This invention relates to a method of controlling the fuel supply to an internal combustion engine at deceleration, and more particularly to a fuel supply control method of this kind, which is intended to improve the driveability of the engine under low atmospheric pressure conditions such as at high altitudes.
An internal combustion engine is liable to emit a large amount of unburnt fuel when the intake pipe pressure becomes low, at deceleration or like low load operation, badly affecting the fuel consumption, the emission characteristics, etc. the engine. Particularly in an internal combustion engine provided with a three-way catalyst for purifying exhaust gases in the exhaust passage, emission of exhaust gases containing a large amount of unburnt fuel causes burning of the catalyst bed to thereby hinder the purification of noxious exhaust gas ingredients. To overcome this disadvantage, it has been proposed by the assignee of the present application to decrease the fuel supply to the engine to lean the air/fuel ratio of the mixture to be supplied to the engine when the engine is operating in a predetermined decelerating operating region (Japanese Provisional Patent Publication (Kokai) No. 57-137633).
According to this proposed method, the intake pipe absolute pressure is used to determine whether or not the engine is operating in the predetermined decelerating region (hereinafter called "the mixture-leaning effecting region"). Specifically, the engine is determined to be in the mixture-leaning effecting region if the intake pipe absolute pressure is lower than a predetermined value (hereinafter called "the leaning-determining value"), which is set in consideration of the temperature of the catalyst bed to be assumed during operation of the engine under standard atmospheric pressure, such as during running at low altitude. If it is determined whether the engine is operating in the mixture-leaning region with reference to the leaning-determining value thus set under standard atmospheric pressure, excessive leaning will be inevitable when the vehicle is running under low atmospheric pressure such as when running at a high altitude, resulting in degraded driveability of the engine and often in engine stalling. That is, the back pressure acting upon the engine is low under low atmospheric pressure, which makes the emission of exhaust gases smooth to thereby reduce the amount of residual gas in the cylinders, hence improve the engine combustibility. The improved combustibility causes reduction in the quantities of hydrocarbon (HC) and carbon monoxide (CO) contained in the exhaust gases, which in turn alleviates the burden on the three-way catalyst, whereby the temperature of the catalyst bed decreases. Therefore, it is possible, under low atmospheric pressure conditions, to set the leaning-determining value to a value lower than a value which is optimal under standard atmospheric pressure, to thereby enable expanding of the O.sub.2 feedback effecting region whereupon the fuel supply quantity to the engine is controlled in a feedback manner in response to oxygen concentration in the exhaust gases such that the air/fuel ratio is maintained at a value that enables ideal combustion of the mixture, e.g. a theoretical air/fuel ratio, whereby the driveability of the engine is improved. Therefore, it is a requisite that the leaning-determining value, which defines the boundary between the O.sub.2 feedback effecting region and the mixture-leaning region, should be set to an optimal lower value under low atmospheric pressure, e.g. during running at high altitudes, than that under standard atmospheric pressure.
On the other hand, when the intake pipe absolute pressure assumes a further lower value than the leaning-determining value, the catalyst bed can sometimes be burned even when leaning of the mixture is effected. In such case, it has been employed to interrupt the fuel supply to the engine (i.e., fuel cut). The fuel cut-determining value, which defines the boundary between the fuel cut-effecting region and the leaning effecting region, is also required to be set to a value lower than a value which is optimal under standard atmospheric pressure, for the same reason as stated with respect to setting of the leaning-determining value. That is, the range of the fuel cut effecting region should be made narrower under low atmospheric pressure, by setting the fuel cut-determining value to a more appropriate lower value, so as to avoid a drop in the engine output or engine torque to be caused by fuel-cut, thereby achieving improved driveability of the engine.